Load-carrying skeletal members, such as a human hip, frequently are rendered non-functional because of fracture, damage, disease, resections for malignancy or disease, or because of pain or malformation. Such members are commonly repaired by total joint replacements with artificial components. One type of bone replacement that has been particularly successful over the past thirty years is that of the human hip. Such hip prostheses typically include a femoral portion or component which is implanted in the femur and an acetabular component which is secured to the pelvis. The femoral component includes a head which rotates in a socket formed in the acetabular component. The femoral component typically has a rectangular or square cross-sectional shape and includes four outer surfaces, the lateral, medial, anterior and posterior surfaces.
Many known hip prostheses require the use of cement for installation of the femoral component into the medullary canal of the femur. One type of cement which is commonly used is a polymethylmethacrylate.
Success of a femoral component of a total hip implant depends in large part On the technical precision with which the implant is inserted. One factor which contributes to the success of a femoral component is centering of the component within the central cavity in the medullary canal of the femur into which the component is inserted. Centering of the component insures that the thickness of the cement mantle surrounding the component is uniform on all sides. Uniformity of the cement mantle renders the load distribution at the bone-cement and metal-cement interfaces generally uniform on all sides of the component, thus avoiding problems associated with overstressing one area of the interface, such as fracturing of the mantle, separation of the mantle from the bone or separation of the component from the mantle. Another factor which has been identified as contributing to the success of either an uncemented or cemented component is proper rotational position of the femoral component about its axis with respect to the femur. Proper rotational position, or anteversion, allows for accurate reproduction of the mechanical orientation of hip joints and produces the desired stability and range of motion.
A third factor believed to contribute to the success of a cemented femoral component is the bond between the cement and the inner surfaces of the cavity in the medullary canal of the femur. This bond can be improved by distributing the cement into the trabecular bone. To achieve such a result, it is a common practice to pressurize the cement prior to insertion of the femoral component.
A fourth factor which recently has been recognized to be crucial to the long term stability of cemented femoral components in total hip replacements is the strength of the bond between the bone cement and metal of the femoral component. The interface between the prosthesis and bone cement has been determined to be the weak link in the mechanical integrity of the femoral component. Conversely, a secure cement-metal interface provides an even load distribution with respect to the surrounding cement, and thus decreases localized loading and reduces the risk of cement fracture. It has been established that debonding of the cement-metal interface is the initiating event in the failure of fixation of cemented femoral components. Fractures in the cement mantle are usually associated with debonding at the cement-metal interface, and radial cement fractures propogate from the cement-metal interface outwardly. Moreover, it has been found that debonding of the cement-metal interface starts in the proximal and distal regions of the mantle and progresses toward the middle of the component.
Recent studies have shown that abundant porosity in the cement at the cement-metal interface is one cause of debonding. In many failed femoral prostheses, there existed a high concentration of pores in the cement at the cement-metal interface relative to the concentration of pores in the bulk cement mantle. This porosity will hereinafter be referred to as "interfacial porosity". In experiments it was observed that interfacial pores formed because air was pulled down along the interface between the metal and the cement. If the cement had had a lower viscosity, it would have fully contacted the surface of the metal and filled in the areas left by displaced air. However, the cement was too viscous to do so. During curing, the pores at the interface expanded as the cement started to heat, and then after the peak temperature is reached, the pores puckered. This heating caused new pores to appear at the interface. These pores were formerly located only a few microns away from the interface and expanded to reach the interface. Such interfacial porosity was not observed to be affected by the type of prostheses, surface finish or by cement centrifugation. Interfacial porosity is believed to be detrimental to the long-term mechanical integrity of the femoral component because it reduces the effective surface area for cement-metal bonding and causes stress concentrations in the cement which may initiate cracks.
This interfacial porosity was determined to be a result of the rheology of the cement during the insertion of the implant (S. P. James, T. P. Schmalzried, F. J. McGarry and W. H. Harris, "Extensive Porosity at the Cement-Femoral Prosthesis Interface: A Preliminary Study", 27 J. Biomed. Mater. Res., 71 (1993)). The observed porosity was aligned with stem geometry and in some specimens was concentrated in the highest stress areas: the proximal and distal portions of the interface. This combination of high stress and extensive porosity is believed to explain why debonding starts in the proximal and distal regions.
It is therefore an object of the present invention to provide a method and apparatus for reducing interfacial porosity in the cement at the cement-metal interface of a femoral component.
It is another object of the present invention to provide a method and apparatus for reducing the porosity in the cement at the metal-cement interface of a femoral component which can be utilized in conjunction with existing prostheses.
It is a further object of the present invention to provide apparatus for reducing the porosity in the cement at the metal-cement interface of a femoral component which can be removed after use without disturbing the position and angular orientation of the component.
It is another further object of the present invention to provide a method and apparatus for centering a femoral component during implantation into a femur.
It is yet another further object of the present invention to provide a method and apparatus for controlling the angular position of the femoral component during insertion thereof into a femur.